WASHINGTON — The main tool available in the search for the missing jet operating as Malaysia Airlines Flight 370 is imprecise and awkward. It also is unfamiliar to the investigators using it, but it is all they have.

They lack the four ordinary ways of finding an airplane: primary radar, which sends out radio signals and listens for echoes that bounce back from objects in the sky; secondary radar, in which the plane answers an electronic query and sends back a message giving its identification number and altitude; automated transmissions from the plane to a maintenance base; and oral reports by radio from the cockpit crew.

The plane was out of range of ground-based radar by the time it disappeared March 8 shortly after takeoff, and the secondary radar and automated transmission systems were not working and possibly turned off. There was no word from the pilots or crew.

What is left is an hourly electronic “handshake,” or digital communication, between the airplane and a satellite orbiting 22,250 miles above the Indian Ocean. The satellite is in a geosynchronous orbit, meaning that it appears to hang in a fixed spot above the Earth.

But the handshake is mostly devoid of data and cannot be used to pinpoint the plane’s last known location. It is the electronic equivalent of catching someone’s eye in the crowd; all it does is establish the angle between the satellite and the plane.

At a news conference Saturday, Malaysia’s prime minister, Najib Razak, released a map showing the two geographic arcs where the last satellite signal sent from the plane could have come from. One arc extended north from Thailand to Kazakhstan, the other south from Indonesia to the southern Indian Ocean, about 1,000 miles west of Australia.

The satellite handshakes formed a circle all the way around the Earth, but only part of the circle — the two arcs — is relevant to the search because the plane would have run out of fuel beyond the end of each arc. U.S. investigators say the southern arc is of more interest.

Mikael Robertsson, a co-founder of Flightradar24, a global aviation tracking service, said the way the plane’s communications had been shut down pointed to the involvement of someone with considerable aviation expertise and knowledge of the air route, possibly a crew member, willing or unwilling.

The Boeing’s transponder was switched off just as the plane passed from Malaysian to Vietnamese air traffic control space, making it more likely that the plane’s absence from communications would not arouse attention, Robertsson said from Sweden.

“Always when you fly, you are in contact with air traffic control in some country,” he said. “Instead of contacting the Vietnam air traffic control, the transponder signal was turned off, so I think the timing of turning off the signal just after you have left Malaysian air traffic control indicates someone did this on purpose, and he found the perfect moment when he wasn’t in control by Malaysia or Vietnam. He was like in no-man’s country.”

The signs indicated involvement of the crew, Robertsson said, but he emphasized that those signs were not definitive.

Xu Ke, a former commercial pilot who has advised the Chinese government, said the details suggested that at least one crew member, most likely one of the pilots, was involved in seizing control of the aircraft, either willingly or under coercion.

“The timing of turning off the transponder suggests that this involved someone with knowledge of how to avoid air traffic control without attracting attention,” Xu said.

Especially since 9/11, Xu said, security on cockpit doors has been reinforced so that forced entry would be difficult without the pilots’ having ample time to send a warning signal.

“We have to be careful about our words and conclusions and examine all the possibilities, but the likelihood that a pilot was involved appears very likely,” Xu said. “The Boeing 777 is a relatively new and big plane, so it wouldn’t be anyone who could do this, not even someone who has flown smaller passenger planes, even smaller Boeings.”

The possible northern corridor Najib described bristles with military radar, making it more likely that the plane either went south or, if it did fly north, did not make it far, Robertsson said.

“I don’t really think that the aircraft could have flown so far over the land, because it would need to pass over so many countries that someone should have picked it up,” Robertsson said. “If they had taken the northern corridor, they could have gone down before they reached land, so it’s also possible.”

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